Gas distributor for vapor coating method and container

ABSTRACT

A method for introducing an inert carrier gas into a coating container used to provide a metallic coating on articles. The method includes introducing the inert carrier gas into the coating container as a plurality of carrier gas streams proximate the top of the coating container. The carrier gas streams are formed and introduced into the coating container before encountering a source material for the metallic coating, and each carrier gas stream is introduced so that the inert carrier gas at least initially moves within the coating container in a circular swirling fashion above and before encountering the source material and the article.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.10/671,193, filed Sep. 25, 2003, now abandoned which is a divisional ofU.S. application Ser. No. 10/029,311, filed Dec. 20, 2001 now U.S. Pat.No. 6,986,814.

BACKGROUND OF THE INVENTION

The present invention relates generally to a gas distributor useful fora vapor coating method and container. The present invention particularlyrelates to a gas distributor for introducing nonoxidizing or inertcarrier gases for vapor coating of articles such as gas turbine engineblades with a metallic coating, especially an aluminide coating.

Certain articles operating at elevated temperatures in an oxidizingatmosphere have been provided with environmental protection in the formof coatings of various types. For example, components such as gasturbine engine turbine blades, vanes and other airfoils operating athigh temperatures typically experienced in the turbine section of theengine frequently include metallic surface coatings alone or in variouscombinations with other materials. Such coatings are capable ofresisting the oxidation, corrosion and sulfidation conditions generatedduring high temperature operation.

Application methods for such metallic coatings include depositing avapor of one or more protective metals, for example aluminum or alloysof aluminum, to provide a form of aluminide coating, on an articlesurface at high temperatures. Such vapor coating methods are typicallyconducted in a nonoxidizing or inert atmosphere (e.g. hydrogen,nitrogen, helium or argon) within a coating container or chambercommonly referred to as a “retort”. Generally, the article or moretypically articles (e.g., airfoils such as turbine blades) to be coatedare placed within the container, along with a source of the aluminidecoating, typically in the form of metallic pellets or powder, and isoften retained in perforated baskets that can be arranged in rows tosurround the articles. The container is then placed within a heater suchas a furnace to generate a coating vapor. Generation of the coatingvapor typically includes the use of halide “activators” such asfluorides, chlorides or bromides. This halide activator can be in theform of a gas that is introduced into the container to react with thesource of the aluminide coating to form the aluminide-bearing gas or canbe generated from a halide activator source within the container thatforms the reactive halide gas upon heating.

The aluminide-bearing gas is typically transported or moved within thecoating container by a nonoxidizing or inert carrier gas (e.g.,hydrogen, nitrogen, helium or argon). In some vapor coating systems,this carrier gas is introduced through the bottom of the container andcarries the aluminide-bearing gas upwardly to coat the articles. See,for example, U.S. Pat. No. 4,148,275 (Benden et al), issued Apr. 10,1979; U.S. Pat. No. 5,928,725 (Howard et al), issued Jul. 27, 1999. Inother vapor coating systems, the carrier gas is introduced through thetop of the coating container and then diffuses throughout the containerto carry the aluminide-bearing gas and coat the articles. See U.S. Pat.No. 6,039,810 (Mantkowski et al), issued Mar. 21, 2000. The advantage inintroducing a carrier gas, such as argon, at the top (versus the bottom)of the container is that argon, being denser and heavier than air, willnaturally flow downwardly through the container to commingle with themetallic (aluminide) coating vapor and will also act as a “plunger” toaid in the internal coating of the articles.

In one such system where the carrier gas is introduced through the topof the container, a gas distributor is used to disperse the carrier gas.One such gas distributor has a configuration similar to that of a“shower head” in that it is provided with a plurality of gas outletholes spaced along the periphery of the cylindrical or disk-shaped headthrough which the carrier gas exits. This “shower head” distributor istypically positioned at the top of the container and above the aluminidegenerating pellets and articles to be coated.

It has been found that when a carrier gas such as argon is introducedthrough such a “shower head” distributor at the top of the container,the aluminide-bearing gas is not consistently moved or mixed within thecoating container. This is particularly true as the argon gas moves anddiffuses through the rows of aluminide generating pellets and throughthe rows of articles (e.g., airfoils) to be coated. Because the rows ofpellets and articles impede or resist the gas flow, regions havingvarying densities of aluminide-bearing gas can be formed, thus creatinga nonhomogeneous environment of the aluminide-bearing gas surroundingthe articles to be coated. This nonhomogeneous environment of thealuminide-bearing gas usually results in an inconsistent distribution ofthe aluminide coating on the exterior of the article, as well asinconsistent internal gas flow and coating of the interior surface ofthe article (e.g. hollow airfoils such as hollow gas turbine blades).

Accordingly, it would be desirable to be able to provide a gasdistributor that can introduce the carrier gas in a manner such that thealuminide-bearing gas is consistently moved and mixed within the coatingcontainer such that a more uniform and consistent aluminide coating isprovided on the exterior of the articles, as well as on the interior ofhollow articles.

SUMMARY OF THE INVENTION

The present invention relates to a gas distributor suitable forintroducing a carrier gas at the top of a coating container used toprovide a metallic coating on articles. This gas distributor comprises:

-   -   (a) a gas inlet;    -   (b) a gas outlet head in communication with the gas inlet for        receiving a flow of gas from the gas inlet and having a        peripheral surface;    -   (c) a plurality of gas outlets spaced along the peripheral        surface, the gas flow exiting as a gas stream from each gas        outlet;    -   (d) a plurality of gas deflectors, each deflector being        proximate to one of the gas outlets and at least initially        directing the gas stream exiting each gas outlet in at least a        generally centripetal path.

The present invention also relates to an apparatus for vapor coating ofarticles with a metallic coating. This apparatus comprises;

-   -   (1) a coating container having a base, a top spaced from the        base, and a side wall connecting the top and the base;    -   (2) the gas distributor previously described for introducing a        carrier gas into the coating container positioned such that the        gas outlet head is proximate the top of the coating container;    -   (3) at least one holder for each article to be coated positioned        within the coating container and below the gas outlet head of        the gas distributor;    -   (4) at least one holder for the source of the metallic coating        positioned within the coating container and below the gas outlet        head of the gas distributor.

The present invention also relates to a method for introducing thecarrier gas into the coating container for vapor coating of articleswith a metallic coating. This method comprises the step of introducingthe carrier gas as a plurality of carrier gas streams proximate the topof the coating container, each carrier gas stream flowing at leastinitially in at least a generally centripetal path.

The present invention further relates to a method for coating thearticles with a metallic coating in the coating container. This methodcomprises the steps of:

-   -   (a) loading the coating chamber of the container with articles        to be coated;    -   (b) loading the coating chamber of the container with a source        of a metallic coating;    -   (c) introducing an inert carrier gas as a plurality of inert        carrier gas streams proximate the top of the coating chamber of        the loaded coating container, each carrier gas stream flowing at        least initially in a curved generally centripetal, downward path        to provide an inert gas atmosphere in the coating chamber of the        loaded container;    -   (d) after the inert gas atmosphere is provided in the coating        chamber, heating the loaded coating container to a temperature        sufficient to form a metallic coating gas from the metallic        coating source;    -   (e) continuing the flow of the carrier gas into the coating        chamber of the loaded container to move the metallic coating gas        within the coating chamber of the loaded container so as to        deposit a coating on the articles.

The gas distributor and vapor coating apparatus, as well as the methodfor introducing the carrier gas, and method for coating the articles, ofthe present invention provides a number of significant benefits,especially when introducing the carrier gas at or proximate the top of acoating container for vapor coating of articles with a metallic coating.Because the carrier gas (e.g., argon) is introduced into the top of thecoating container at least initially in at least a generally centripetalpath, this carrier gas tends to move in circular or swirling fashion andthus keeps the environment above the articles to be coated more uniformand homogeneous. As a result, the environment of the metallic coating(e.g., aluminide)-bearing gas surrounding the articles tends to be moreuniform and homogeneous, thus leading to a more uniform metallic coatingon the exterior surface of the articles. In addition, in the case ofhollow articles, such as airfoils, there will be a more uniformdistribution of gas flow internally, resulting in a more uniformmetallic coating on the interior surface of the articles.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is perspective view of an embodiment of the gas distributor ofthe present invention is useful.

FIG. 2 is bottom view of the distributor of FIG. 1

FIG. 3 is sectional side view of an embodiment of a vapor coatingapparatus using the distributor of FIG. 1.

FIG. 4 is a sectional view taken along line 4-4 of FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings, FIGS. 1 and 2 show an embodiment of the gasdistributor indicated generally as 10. Distributor 10 comprises agenerally cylindrical hollow gas inlet tube or pipe 14 for receiving thegas from a source of supply (not shown) and a generally cylindrical ordisk-shaped gas outlet head or manifold indicated as 18 connected topipe 14. As shown in FIG. 1, pipe 14 is provided with a hole indicatedas 20 for securing the source of the gas to pipe 14. Although not shown,manifold 18 is also hollow so that as gas is fed into pipe 14, this gasis then delivered to manifold 18, i.e., pipe 14 is in fluidcommunication with manifold 18.

Referring especially to FIG. 1, manifold 18 comprises a bottom surfaceindicated as 22 and is shown as having a generally circular peripheralsurface indicated as 26. However, peripheral surface 26 can have othershapes or configurations, including polygonal shapes or configurations(e.g., hexagonal, octagonal, decagonal, dodecagonal, etc.) A pluralityof gas outlets in the form of apertures or holes 30 are formed in andspaced along peripheral surface 26. The number of holes 30 can varydepending on the size of the holes and the size of peripheral surface26. Usually, the number of holes 30 along peripheral surface 26 is atleast 4, and is typically in the range of from 4 to 20, and moretypically in the range of from 6 to 12.

Proximate each of the holes 30 is an angular gas baffle or deflector 34which is shown in FIG. 1 as having an open generally trapezoidal or“hooded” configuration or shape. However, deflectors 34 can also beformed to have other configurations or shapes (e.g., rounded). Eachdeflector 34 is shown as comprising a generally triangular aft deflectorcomponent 36 having a generally forward deflecting inner surface 38 anda generally triangular upper deflector component 40 having a generallydownward deflecting inner surface 42. Surfaces 38 and 42 of components36 and 40 intersect along a seam or edge 46. As shown in FIG. 1, as thegas flow or stream exits each hole 30, it is at least initiallydeflected by inner surface of 38 (of component 36) into a generallycentripetal path (i.e., along or parallel to surface 26) and by innersurface 42 (of components 40) into a slightly downward path (i.e.,eventually away from bottom surface 22). As a result, the gas streamexiting from holes 30 moves into a curved generally centripetal,slightly downward path, as indicated by arrows 50.

As shown in FIG. 3, gas distributor 10 is typically used with a vaporcoating apparatus indicated generally as 100 that includes a generallycylindrical coating container indicated as 110. As shown in FIG. 3,distributor 10 (including pipe and manifold 18) is sized to fit withincontainer 110. Container 110 has a top or lid indicated as 114, a baseindicated as 118 spaced from lid 114, and a generally cylindricalcircumferential side wall indicated as 122 that connects lid 110 andbase 118 and extends downwardly beyond base 118. Lid 114, base 118 andside wall 122 of container 110 define an interior coating chamberindicated as 124. As also shown in FIG. 3, pipe 14 of distributor 10 isinserted partially through a hole or aperture 126 at or proximate thecenter of lid 114; manifold 18 is positioned within chamber 124 at orproximate the top thereof, i.e., proximate lid 114.

Apparatus 100 also has an article support or holder 128 attached to orotherwise associated with base 118 of container 110 that is providedwith apertures, typically in the form of slots (not shown) or othersuitable devices, for receiving and holding articles such as airfoils(e.g., turbine blades) 132 to be coated. Apparatus 100 also has holdersin the form of perforated baskets indicated as 140 positioned withincontainer 110 for receiving or holding pellets of the metallic coating.As shown in FIG. 3, baskets 140 and articles 132 are below manifold 18of distributor 10. The number and spacing of baskets 140 and articles132 can be varied depending upon the internal dimensions andconfiguration of container 110, the size of articles 132 to be coatedand like factors known to those skilled in the art. A representativearrangement is shown in FIG. 4, where articles 132 and baskets 140 arearranged in alternating concentric rows or circles. The spacing of therows of articles 132 and baskets 140 should be such as to allow the freeflow of gas therebetween. Between each row of articles 132 and baskets140 is typically placed discrete portions of a powdered halide activatorindicated generally as 146. This powdered halide activator is typicallyplaced so as not to touch or be in contact with articles 132 or baskets140.

In order to coat hollow articles 132 (e.g., airfoils such as turbineblades), having internal surfaces and passageways in predeterminedlocations with the aluminide coating, it may be necessary to mask thoseareas not requiring any coating. After loading holder 128 with thearticles 132, the container 110 and its contents, which also contains ametallic coating source (e.g., aluminum pellets) loaded into baskets140, is sealed and then loaded into a furnace or other heating device.Gas inlet pipe 14 is then connected to as source of a nonoxidizing orinert carrier gas such as hydrogen, nitrogen, helium, or argon.

After loading the container 110 into a furnace or other heating device,interior chamber 124 is purged of air by introducing the nonoxidizing orinert carrier gas through gas inlet pipe 14 which then flows intomanifold 18 and exits through gas outlets 30 as gas streams 50 so as toprovide an inert gas atmosphere. The rate at which the carrier gas flowsinto pipe 14 (and out of holes 30 as gas streams 50) of manifold 18 isusually at least about 15 ft³/hour (about 425 liters³/hour), and istypically in the range of from about 15 to about 120 ft³/hour (fromabout 425 to about 3,398 liters³/hour), and more typically from about 40to about 70 ft³/hour (from about 1133 to about 1982 liters³/hour). Asthe gas exits outlets 30, each of gas streams 50 are directed bydeflectors 34 into a curved generally centripetal, slightly downwardpath so that the inert carrier gas swirls above the concentric rows ofbaskets 140 and articles 132, thus creating a relatively uniform andhomogeneous atmosphere in chamber 124. In addition, the pressure of thegas flow forces the streams 50 of the carrier gas downwardly fromdistributor 10 and around and through the rows of baskets 140 andarticles 132.

When this inert gas atmosphere is provided or established, container 110is usually heated to an elevated, preselected temperature, of at leastabout 1000° F. (about 538° C.), typically in the range from about 10000to about 2200° F. (from about 5380 to about 1204° C.), and moretypically in the range of from about 1900° to about 2000° F. (from about1038° to about 1093° C.). The particular elevated temperature selectedwill depend on the coating application parameters desired (including thesource of metallic coating used) and other factors that would beunderstood by those skilled in the art. Upon reaching this preselectedtemperature, the powdered activator 146 will form a reactive halide gas.Suitable halide activators can be selected from aluminum chloride,aluminum fluoride, ammonium fluoride and mixtures thereof. This reactivehalide gas flows through the pellets in baskets 140 containing themetallic coating source (e.g., aluminum source) and reacts with thealuminum source to provide the metallic coating gas in the form of analuminum halide or aluminide-bearing gas. The aluminum source can be anyaluminum or aluminum alloy, for example, cobalt aluminum alloys (CoAl),iron aluminum alloys (FeAl), or chromium aluminum alloys (CrAl),typically in powder or pelletized form. As would be understood by thoseskilled in the art, the reaction kinetics controlling the rate offormation of the aluminide-bearing gas will be dependent on thetemperature, as well as the rate at which the carrier gas is introducedinto chamber 124 by distributor 10 which is the driving force (i.e.,“plunger”) for moving the aluminide-bearing gas within chamber 124, aswell as amongst, around and through articles 132. This, in turn controlsthe rate of deposition of the coating upon articles 132 and hence thecoating thickness.

As the aluminide-bearing gas flows over the surfaces of articles 132, aswell as through the holes in articles 132, such air cooling holes (notshown) in the case of a hollow airfoil, the aluminide-bearing gas isreduced to aluminum, thereby coating the exterior surfaces of articles132, as well as the interior surfaces of hollow articles 132. Of course,the rate and uniformity of deposition is greatly influenced by theuniformity of the aluminide-bearing gas environment in proximity toarticles 132, which is in turn controlled by the rate at which thecarrier gas is introduced into chamber 124 and mixes with thealuminide-bearing gas, as previously discussed.

In order to force the aluminide-bearing gas through the rows of articles132, a certain minimum pressure of the carrier gas is required. This istypically achieved by having the carrier gas continue to flow intochamber 124 at the previously indicated flow rates through pipe 14.Thus, the carrier gas can not only be used to control the uniformity ofthe aluminide-bearing gas environment, and hence reduction of thealuminide-bearing gas at the surface (exterior and interior), but it canalso be balanced to provide the necessary pressure to move and force thealuminide-bearing gas through the rows of articles 132 (and into theinterior when articles 132 are hollow), thereby coating them. Inparticular, the inert carrier gas commingles and mixes with thealuminide-bearing gas and acts, in essence, as a “plunger” to aid in thecoating of external (and internal) surfaces of articles 132. Afterpassing through articles 132, the remaining aluminide-bearing gas isexhausted from chamber 124 through gas exhaust outlet indicated as 152and into an open evacuation chamber or area indicated as 160 defined bythe extension of side wall 122 beyond base 118. Upon completion of thecoating operation to the desired coating thickness, container 110 can beremoved from the furnace and cooled or optionally furnace cooled, whilemaintaining an inert gas atmosphere if desired.

While specific embodiments of the method of the present invention havebeen described, it will be apparent to those skilled in the art thatvarious modifications thereto can be made without departing from thespirit and scope of the present invention as defined in the appendedclaims.

1. A method for introducing a carrier gas into a coating container forvapor coating of at least one article with a metallic coating, themethod comprising: flowing a carrier gas into a gas outlet head having aperipheral surface; causing the carrier gas to exit the gas outlet headand enter the coating container as multiple gas streams through aplurality of gas outlets spaced along the peripheral surface; and thendeflecting the gas streams after exiting the gas outlets with aplurality of gas deflectors, each deflector deflecting a correspondingone of the gas streams exiting a corresponding one of the gas outletsprior to the gas stream encountering a source material for the metalliccoating.
 2. A method for introducing an inert carrier gas into a coatingcontainer for vapor coating of at least one article with a metalliccoating, the coating container having a base, a top spaced from thebase, and a side wall connecting the top and the base, the methodcomprising the step of introducing the inert carrier gas into thecoating container as a plurality of carrier gas streams proximate thetop of the coating container, the carrier gas streams being formed andintroduced into the coating container before encountering a sourcematerial for the metallic coating, each carrier gas stream beingintroduced so that the inert carrier gas at least initially moves withinthe coating container in a circular swirling fashion above and beforeencountering the source material and the article.
 3. The method of claim2 wherein each carrier gas stream is individually introduced into thecoating container through an opening in a surface and then deflectedafter exiting the opening and prior to encountering the source material.4. The method of claim 3 wherein the inert carrier gas is introduced ata gas flow rate of at least about 15 ft³/hour (about 425 liters³/hour).5. The method of claim 4 wherein the gas flow rate is in the range offrom about 15 to about 120 ft³/hour (from about 425 to about 3,398liters³/hour).
 6. The method of claim 5 wherein the gas flow rate is inthe range of from about 40 to about 70 ft³/hour (from about 1133 toabout 1982 liters³/hour).
 7. The method of claim 4 wherein the inertcarrier gas is argon.
 8. The method of claim 2 wherein each carrier gasstream is individually introduced into the coating container through anopening in a surface and then deflected in a direction approximatelyparallel to the surface prior to encountering the source material.
 9. Amethod for coating an article with a metallic coating in a vapor coatingcontainer having a base, a top spaced from the base, and a side wallconnecting the top and the base, the base, top and side wall defining acoating chamber, the method comprising the steps of: (a) loading thecoating chamber of the container with at least one article to be coated;(b) loading the coating chamber of the loaded container with a source ofa metallic coating; (c) introducing an inert carrier gas into thecoating chamber as a plurality of inert carrier gas streams proximatethe top of the loaded container, the carrier gas streams being formedand introduced into the coating chamber before encountering the sourceof the metallic coating, each carrier gas stream being deflected beforeencountering the source material so that the inert carrier gas at leastinitially moves within the coating chamber in a circular swirlingfashion above and before encountering the source material and thearticle to provide an inert gas atmosphere in the coating chamber of theloaded container; (d) after the inert gas atmosphere is provided in thecoating chamber of the loaded container, heating the loaded container toa temperature sufficient to form a metallic coating gas from themetallic coating source; and then (e) continuing the flow of the inertcarrier gas into the coating chamber of the loaded container to move themetallic coating gas within the coating chamber of the loaded containerso as to deposit a coating on the article.
 10. The method of claim 9wherein the article is an airfoil turbine blade, wherein the metalliccoating source is an aluminum source and wherein the metallic coatinggas is an aluminide-bearing gas.
 11. The method of claim 10 whichcomprises the further step of loading the coating chamber of thecontainer with a halide activator prior to step (d) and wherein thehalide activator forms a reactive halide gas after heating during step(d) that reacts with the aluminum source to form the aluminide-bearinggas.
 12. The method of claim 10 wherein the halide activator is selectedfrom the group consisting of aluminum chloride, aluminum fluoride,ammonium fluoride and mixtures thereof.
 13. The method of claim 11wherein the inert carrier gas is introduced during step (e) into thecoating chamber of the loaded container at a gas flow rate of at leastabout 15 ft³/hour (about 425 liters³/hour).
 14. The method of claim 13wherein the gas flow rate during step (e) is in the range of from about15 to about 120 ft³/hour (from about 425 to about 3,398 liters³/hour).15. The method of claim 14 wherein the gas flow rate during step (e) isin the range of from about 40 to about 70 ft³/hour (from about 1133 toabout 1982 liters³/hour).
 16. The method of claim 13 wherein the inertcarrier gas is argon.
 17. The method of claim 13 wherein the loadedcontainer is heated during step (d) to a temperature of at least about1000° F. (about 538° C.).
 18. The method of claim 17 wherein the loadedcontainer is heated during step (d) to a temperature in the range fromabout 1000° F. to about 2200° F. (from about 538° C. to about 1204° C.)19. The method of claim 18 wherein the loaded container is heated duringstep (d) to a temperature in the range of from about 1900° F. to about2000° F. (from about 1038° C. to about 1093° C.).
 20. The method ofclaim 9 wherein each carrier gas stream is individually introduced intothe coating container through an opening in a surface and then deflectedin a direction approximately parallel to the surface prior toencountering the source material.